Updated by: Arabinda Naik
They are considered as true fishes and includes three main classes. Class-Cyclostomata includes the round mouthed fishes. They are also called jawless fishes. They are marine but show migration to freshwater for spawning. Adult die within few days after spawning but their larvae return to the ocean. The body of cyclostomes is elongated, smooth has paired fins and are without scales. The cranium and vertebral column are cartilaginous. Circulation is of closed type. They have 6-15 pairs of gill slits for respiration. e.g., petromyzon(lamprey), Myxine (hagfish).
Describe General Characteristics of Class Chondrichthyes
(Cartilagenous Fishes) (Chandros-carrilagr; ichthys— fish) have over 600 species including sharks. rays and skates. They are called cartilaginous fishes due to the presence of cartilaginous skeleton. they are mostly marine called poikilothermic animals (i.e., they have the capacity to regulate their temperature). The body is laterally compressed, and spindle shaped. Notochord is persistent throughout life. Mouth is ventral in position, skin is tough containing minute placoid scales and teeth arc modified placoid scales. They have Strong jaw and are predaceous by nature. Gill slits are generally five pairs and gill cover (operculum) are absent. Heart is two chambered with one auricle and one ventricle. Some of them possess electric organs (e.g., Torpedo) and some possess poison sting (e.g., trygon). Paired and unpaired fins are present. which are not supported by fin rays. The tail fin is always heterocercal (unequal lobes). They are called ureotelic animals as they secrete urea. Sexes are separate and fertilisation is internal. Males usually have claspers on pelvic fins, which help in copulation. Many of them arc viviparous. Scoliodon (shark or dog fish), Carcharodon (great white shark), Pristis (saw fish), Trygon (stingray), Sphyrna (hammer headed shark).
Describe General Characteristics of Class Osteichthyes
(Bony Fishes) The class—Osteichthyes (osteon — ichthys — fish) includes bony fishes such as carps, catfishes, sardines, mackerels, seahorse, etc. There are 25000 species of fishes found all over the world. They have a bony endoskeleton. They are found in marine water as well as in freshwater. The body is streamlined to facilitate easy movement through water. Exoskeleton contains dermal scales like cycloid or ctenoid Gills are covered with (gill cover) and are generally four pairs. Heart is two-chambered with one auricle and one ventricle. Air bladders are present. They are the hydrostatic organs. Fins are paired and unpaired, which are supported by fin rays. fin is always homocercal (equal or diphycercal type. These arc ammoniotelic animals that excrete ammonia. Sexes are separate. Fertilisation is usually external, mostly viviparous and development is direct.